The present invention relates to an adaptive equalization circuit implemented by a digital circuit and, more particularly, to an adaptive equalization circuit that is suitable for equalization of a non-linear signal, and performs high-order partial response equalization so that an equalization error of a reproduced signal having non-linear distortions is minimized when reproducing it from a high-density recording medium, thereby improving the characteristics of the reproduced signal.
In an apparatus for recording and reproducing data in/from recording media, reproduction of data is carried out without being affected by non-linear distortions, by using a method of detecting non-linear distortions from a waveform and correcting the waveform with an equalizer to remove the non-linear distortions, or a method of controlling the slice level of maximum likelihood decoding.
That is, in the recording/playback apparatus, when the reproduced signal has a waveform with non-linear distortions, an adaptive equalization method is employed, in which the distortions of the waveform are estimated from the reproduced signal to decide the characteristics of the equalizer. To be specific, filter coefficients of the equalizer are optimized so that the second power error between the level after the equalization and the original level of the reproduced signal is minimized.
Further, when equalizing the reproduced signal during playback of recorded data from the recording medium, partial response equalization is employed to suppress enhancement of the high frequency band components of the reproduced signal characteristics and to prevent the error rate from being increased due to noise.
The partial response equalization is a method of provisionally deciding multiple values by appropriately controlling the quantity of interference between codes in a signal, and restricting the signal power with respect to the frequency instead of the decision.
That is, a target signal to be equalized and a signal obtained by delaying the target signal are superposed to make the multi-valued levels easy to appear, and the signal is decoded by performing provability calculation using a Viterbi decoder or the like. Thereby, the levels of the signal are easily detected without using the high frequency band components of the signal.
A conventional equalizer performing such partial response equalization has the following characteristics. That is, when the equalizer receives a reproduced signal from a portion corresponding to an edge of a recording pit of an optical magnetic recording medium or a portion where the direction of vertical magnetization is inverted, it equalizes the reproduced signal to an equalization target value. Therefore, equalization to the target value is not compulsory performed on a portion of the waveform amplitude where the codes are continuous, excluding the both edges of the portion, and thus the high frequency components included in the reproduced signal are prevented from being emphasized unnecessarily, whereby the noise included in the input signal of the equalizer is prevented from being transmitted to the output signal of the equalizer.
Further, when the characteristics of the reproduced signal vary, the SN ratio of the output signal from the equalizer can be maintained by adaptively controlling the characteristics of the equalizer.
This equalizer uses only its output corresponding to the code-inverted portion of the reproduced waveform of the optical disk as a signal for controlling tap coefficients of the equalizer, and adaptively controls the tap coefficients so that the output from the equalizer is partial-response-equalized.
An example of a conventional equalizer as described above is shown in FIG. 2.
An adaptive equalizer shown in FIG. 2 is one disclosed in, for example, Japanese Published Patent Application No. Hei. 8-153370, and this equalizer detects a position corresponding to a pit edge of a reproduced signal or a portion where the direction of magnetization is inverted in vertical magnetic recording, and performs equalization to predetermined reference amplitudes {xe2x88x921,0,+1} at the detected position. Reference amplitudes corresponding to other positions than mentioned above are not defined.
This equalizer has three values xe2x80x9cxe2x88x921xe2x80x9d, xe2x80x9c0xe2x80x9d, and xe2x80x9c+1xe2x80x9d as reference amplitudes. In FIG. 2, 27 denotes an input terminal for receiving a signal to be subjected to waveform equalization, and 12a, 12b, and 12c denote delay means which are connected in series in this order, and each delay means delays its input signal by one unit time T. The signal outputted from the input terminal 27 is applied to the delay means 12a. Further, 25a, 25b, and 25c denote correlators for correlating the output signals from the delay means 12a, 12b, 12c with an output signal from a switch 24 described later, respectively. Further, 26a, 26b, and 26c denote integrators for integrating the output signals from the correlators 25a, 25b, and 25c, respectively.
Further, 20 denote a transversal equalization circuit. In the transversal equalization circuit 20, 12d and 12e denote delay means which are connected in series in this order, and each delay means delays its input signal by one unit time T. The signal outputted from the input terminal 27 is applied to the delay means 12d. Further, 16a, 16b, and 16c denote buffers as multipliers. These buffers 16a, 16b, and 16c receive, as control signals, the output signals from the integrators 26a, 26b, and 26c, and receive, as input signals, the input signal to the delay means 12d, the output signal from the delay means 12d, and the output signal from the delay means 12e, respectively. 14a denotes an adder for adding the output signals from the buffers 16a and 16b, and 14b denotes an adder for adding the output signals from the adder 14a and the buffer 16c. 
Furthermore, 28 denotes an output terminal for outputting the output signal from the adder 14b, that is, the signal which has been subjected to waveform equalization by the adaptive equalizer; 21 denotes a ternary decision circuit for subjecting the signal R from the output terminal 28 to ternary decision; 22 denotes a reference amplitude generation circuit for generating a signal D having a reference amplitude on the basis of the output signal from the ternary decision circuit 21; 17 denotes a subtracter for subtracting the signal R at the output terminal 28 from the output signal D of the reference amplitude generation circuit 22; 29 denotes a delay circuit for delaying an error signal E1 outputted from the subtracter 17 by one unit time T; 24 denotes a switch for disconnecting the output of the delay circuit 29 and generating an error signal E2 to be supplied to the correlators 25a, 25b, and 25c; and 23 denotes an error signal selection circuit for outputting a selection signal S for controlling the switch 24, on the basis of the output from the ternary decision circuit 21.
Next, the operation will be described. The signal, which is obtained by subjecting the output signal R from the transversal equalization circuit 20 to ternary decision by the ternary decision circuit 21, is converted to a ternary signal D having a reference amplitude by the reference amplitude generation circuit 22. The output signal R and the ternary signal D are input to the subtracter 17, and an output error signal E1 is taken out.
The error signal selection circuit 23 extracts, from the output signal of the ternary decision circuit 21, the timing at which an effective error signal is output, and outputs a selection signal S. The switch 24 is operated by the selection signal S so as to send only the effective error signal as a reference error signal E2 to the correlator 25. When the selection signal S becomes active, the switch 24 is closed, whereby the input E2 to the correlator becomes equal to E1. As a result, the tap coefficients of the transversal equalization circuit 20 are adaptively controlled according to the correlation between the reference error signal E2 and the input signal from the input terminal 27.
On the other hand, when the selection signal S is inactive, the switch 24 is opened, and the reference error signal E2 to be input to each correlator 25 becomes xe2x80x9c0xe2x80x9d. So, the values of the tap coefficients of the multiplier 16 in the transversal equalization circuit 20 are not changed.
There are two output signals from the ternary decision circuit 21, and it is assumed that these signals are T1 and T2.
These output signals T1 and T2 from the ternary decision circuit 21 may have any of the following three states according to the level of the input R: both T1 and T2 being inactive, only T1 being active, and both T1 and T2 being active. The reference amplitude generation circuit 22 generates a ternary signal D having a reference amplitude according to the state of the output signals T1 and T2 from the ternary decision circuit.
The error signal selection circuit 23 decides whether the error signal E1 is to be used as a reference error signal E2 or not. When the output signal T1 from the ternary decision circuit 21 is inactive for successive three or more times or when the output signal T2 from the ternary decision circuit 21 is inactive for successive three or more times, the selection signal S to the switch 24 becomes inactive to exclude the error signal E1 from the reference error.
In this construction, since there is a delay equivalent to unit time T in the error signal selection circuit 23, a delay means 29 is needed between the subtracter 17 and the switch 24 as shown in FIG. 2. Further, with this delay, the delay in the equalizer input signal to be input to the correlator 25 is increased by time T.
As an example, a description is given of equalization characteristics obtained as the result of adaptively controlling the equalizer characteristics such that the second power of a difference between the equalizer output value and any of the three equalization target values {xe2x88x921,0,+1} of PR(1,1) is minimized for only the edge of the recording pit, during optical recording in the case where the amplitude of the reproduced signal is lowered due to high recording density. The PR(1,1) means that, when performing partial response equalization, a weight of xe2x80x9c1xe2x80x9d is given to each of the original signal and the signal obtained by delaying the original signal by one unit time.
In this case, although the equalizer has three equalization target values, two values larger in level than the three target values appear in the equalizer output and, therefore, the equalizer output is distributed concentrating on the five equalization target values in total.
On the other hand, as another example, when the equalizer characteristics are adaptively controlled such that the equalization target values are set at {xc2x11} and a difference between the equalizer output value and one of these two values is minimized, two values appear in the equalizer output in addition to the two target values, resulting in four values of reference amplitudes.
Although the low-order partial response method is used to equalize the reproduced signal to the five or four values of reference amplitudes which are not necessarily equal to the reference amplitude of the partial response characteristics, actually equalization is carried out as if using the high-order partial response method. Therefore, the gain of the high frequency band component of the reproduced signal is reduced as compared with that of the typical PR (1,1) equalizer. Having such frequency characteristics, the reproduced signal power which is concentrated on the low frequency band during high density recording can be efficiently extracted and removed without emphasizing the noise in the high frequency band, whereby the error rate of the equalizer output is improved.
In the following maximum likelihood decoder such as a Viterbi decoder to which the equalizer output is applied, the recorded data is decoded with improved error rate, by the method of controlling the slice level using the equalizer output as the reference amplitudes.
By the way, since equalization using a transversal filter is performed to remove linear distortions, waveform distortions comprising only linear distortions are effectively removed from the reproduced signal by equalization using a transversal filter. However, there may be a case where the waveform distortions cannot be effectively removed, depending on the signal waveform.
For example, a reproduced waveform 60 having asymmetry as shown in FIG. 6 corresponds to this case. In the reproduced waveform 60, the convex portions of the reproduced waveform correspond to recording pits 61, and the concave portions thereof correspond to non-recording pits. When this signal is reproduced with a magnetic head, the amplitude level of the waveform of the reproduced signal reaches the saturation level when the recording pit 61 or the interval between the recording pits 61 is relatively long, while the amplitude level of the waveform of the reproduced signal becomes smaller than the saturation level 62 when the recording pit 61 or the interval between the recording pits 61 is relatively short, resulting in non-linear distortions in the reproduced signal.
Further, when performing high-density recording on an optical disk as shown in FIG. 7, a recording pit 75 is continuously formed by a laser beam 70. At this time, in the recording pit 75, the irradiation time with the laser beam 70 varies from portion to portion, whereby the area in the recording pit 75 is not recorded at a uniform level, resulting in unevenness in the recorded signal level in the pit area.
When the recorded signal is reproduced with the laser beam 70, the reproduced waveform has non-linear distortions in a portion 74 where the recorded signal level is uneven. The non-linear distortion components included in the waveform having the non-linear distortions cannot be removed by equalization using a transversal filter. Therefore, waveform equalization is not carried out satisfactorily, resulting in increased error rate.
In the conventional recording/playback apparatus, for the reason described above, non-linear distortions occur in the reproduced signal when the recording density is high. Further, since noise is included in the reproduced signal, when the level of recorded data is detected from the reproduced signal whose amplitude is degraded, incorrect level is detected, resulting in considerable increase in the error rate.
Since an equalizer is usually composed of constituents such as signal delay units, adders, and multipliers, when it performs equalization on a signal having non-linear distortions, it cannot remove the non-linear components. Therefore, the error between the output of the equalizer and the equalization target value varies under influence of the non-linear characteristics, whereby the equalization efficiency is reduced, and the deviation of the waveform from the equalization target value is increased. As the result, it is difficult to equalize the signal waveform to the equalization target value.
Further, when the equalizer performs partial response equalization on a reproduced signal from a high-density recording medium, since the higher harmonic components of the reproduced signal are emphasized, the noise of the high frequency band characteristics having low amplitude, which is included in the reproduced signal, is amplified, whereby the signal after the equalization is deteriorated. As the result, there is the possibility that the output signal from the equalizer may include an error signal.
Further, the conventional equalizer performs equalization on the reproduced signal having non-linear distortions in high-density recording, by using low-order partial response equalization. However, with respect to the PR (1,1), since fixed values like xe2x80x9cxe2x88x921xe2x80x9d, xe2x80x9c0xe2x80x9d, and xe2x80x9c+1xe2x80x9d are used as equalization target values, it is difficult to reset the equalization target values to those suited to waveform equalization. Therefore, it is difficult to perform adaptive equalization of the reproduced waveform with higher precision.
The present invention is made to solve the above-described problems and has for its object to provide an adaptive equalization circuit suited to a non-linear signal, which can perform highly precise adaptive equalization and improve the error rate.
In order to solve the above-described problems, an adaptive equalization circuit according to the invention of claim 1 equalizes an input signal having non-linear distortions, and comprises a linear equalization means for subjecting the input signal having non-linear distortions to high-order partial response equalization adapted to the input signal; a provisional decision circuit for receiving, as an input signal, the output signal from the linear equalization means, and estimating an equalization target value for performing equalization without being affected by the non-linear distortions of the input signal; an error detection circuit for detecting an error between the provisionally decided equalization target value obtained from the provisional decision circuit and the output signal from the linear equalization means; an input distortion detection circuit for detecting an error between the provisionally decided value obtained from the provisional decision circuit and the input signal; an output distortion detection circuit for monitoring the error outputted from the error detection circuit; an equalization target control means for controlling the equalization target value from the provisional decision circuit so that the equalization error is minimized, on the basis of the signals detected by the error detection circuit, the input distortion detection circuit, and the output distortion detection circuit; and a tap coefficient control circuit for controlling tap coefficients of the linear equalization means on the basis of the error detected by the error detection circuit.
Thereby, in the high-order partial response equalization, the non-linear distortions possessed by the signal before the equalization and those possessed by the signal after the equalization are observed quantitatively, and an equalization target value at which the equalization error is minimized is automatically set on the basis of the values, whereby the partial response equalization adapted to the input signal having non-linear distortions is realized. Therefore, even a reproduced signal having non-linear distortions can be equalized with high precision by using a transversal filter that is a linear equalization system, resulting in improved error rate.
Further, according to the invention of claim 2, in an adaptive equalization circuit as described in claim 1, the provisional decision circuit comprises a binary decision circuit for deciding that the output signal from the linear equalization means is either xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d; an addition circuit for subjecting the signal obtained by the binary decision circuit to calculation based on high-order partial response type addition, to obtain how many equalization target values exist; and an equalization target value selection circuit for selecting an appropriate equalization target value from the prepared equalization target values, on the basis of the signal obtained by the addition circuit.
Thereby, it is detected in advance that there are five equalization target values, and adaptive equalization is controlled according to these values. Further, an appropriate equalization target value can be selected from the updated equalization target values, whereby an equalization target value for equalization can be estimated without being affected by the input signal having non-linear distortions.
Further, according to the invention of claim 3, in an adaptive equalization circuit as described in claim 1, the equalization target control means updates the plural equalization target values at the same time or updates every other equalization target values when controlling the equalization target value so as to minimize the equalization error.
Thereby, the equalization target values can be controlled so as to minimize the equalization error, without being affected by non-linear distortions included in the input signal.